Cryostat

DOE-STD-6003-96

Design features that discourage or prevent hydrogen fires and detonations include

(1) leak tight primary confinement to prevent out-leakage of tritium to the secondary confine-

ment, (2) inert gas in the space between primary confinement and secondary confinement

barrier walls, (3) monitors to detect tritium out-leakage or oxygen in-leakage, (4) minimization of

ignition sources or high temperatures near the primary or secondary confinement barriers,

(5) utilization of National Fire Protection Association (NFPA) rated enclosures for electrical

equipment with potential for contact with flammable mixtures. Additional guidance is provided in

Section 6.1.3.10.

Fire Protection

Because fire oxidizes elemental tritium to tritium oxide, a form with a much greater biolog-

ical hazard, design should place high priority on preventing fires. The effect of fire suppression

systems on the facility and the environment should be considered. Chemicals may have a

deleterious effect on tritium cleanup systems, while water may present waste disposal difficul-

ties. Because of the natural affinity of tritium for water and the increased biological hazard of

tritiated water, the use of water as a tritium fire extinguisher should require a technical or eco-

nomic justification. Facilities that have the potential for introducing fire suppression water into a

tritium-contaminated environment should provide a tritiated water collection system with the

capacity to store the total volume of fire suppression run-off. Design should provide for facilities

to dispose of any tritiated water in an environmentally acceptable manner. Additional guidance

is provided in Section 6.1.3.9.

System Cleaning

Design should provide for cleaning of tritium systems before and after installation. Tritium

systems should be able to withstand vacuum conditions necessary for cleaning purposes and

elevated temperatures during bakeout if required prior to equipment removal. Once tritium has

contaminated the primary confinement, only limited cleaning is permissible for tritium-wetted

surfaces.

6.2.1.3 Cryostat

The main function of the cryostat is to provide a vacuum region for thermally insulating

the superconducting coils surrounding the vacuum vessel from the normal building environment.

Based on the safety analysis process, it may be necessary to assign the public safety function

of confinement to the cryostat since it naturally encloses the vacuum vessel. Thus, the cryostat

can be a confinement system for in-vessel radioactive and toxic materials. It could be a primary

confinement if no credit is taken for the vacuum vessel confinement ability, or it could be a sec-

ondary confinement if the cryostat barrier is needed to meet evaluation guidelines. It can also be

a secondary confinement barrier for piping and tubing containing tritium or other radionuclides

that penetrate and are inside the cryostat boundary. It could be a primary confinement boundary

for in-vessel radioactive and toxic materials if the vacuum vessel is opened for maintenance or

inspection. The cryostat is normally a metal chamber surrounding the fusion device which pro-

vides a thermal barrier to conduction and thermal radiation between the superconducting coils

and other cold structures and the fusion building. It may also serve as part of the biological